This web page describes a
justification for ground-based measurements of Vesta using
the Dawn Framing Camera (FC) filters. When Vesta
observations were made the calibration of the FC filters
was suspect, so an empirical calibration was made based on
the Johnson-Cousins standard star system. Because the FC
bands (#2 through #8) had different effective wavelengths
and widths this method of calibration was subject to
uncertainties that could not be straight-forwardly
determined. It is possible to re-calibrate the Vesta
results using ground-based measurements of Vesta with a
duplicate set of the FC filters. This will entail creation
of new magnitude system for each of the 7 narrow band FC
filters, with carefully determined conversion coefficients
for magnitude to flux equations. Vesta flux [watts/m2/nm] will be
determined as a function of Vesta phase, which will
provide the needed comparison with Dawn FC
observations of Vesta versus phase angle.
As a bonus, these ground-based measurements will produce
Vesta albedo vs. wavelength (without the need of
normalizing). These characterizations of the FC filters
should also provide confirmation of the recently-published
in-flight re-calibration of the Framing Cameras prior to
orbit of Ceres.

A listing of the filter pass-bands
(prior to CCD QE response) is given below.

Figure 2.Listing of filter pass-band center wavelength
and width (columns 2 and 3). The effective wavelength column is
for a sun-like star.

Are any of the FC pass-band locations similar enough to the
Johnson-Cousins pass-band locations to enable stars with known JC
magnitudes to be used for calibrating FC measurements?

Figure 3.FC filter pass-band locations in relation to
the Johnson-Cousins pass-band locations. (All response
functions are for use of a ground-based telescope and a specific
CCD, to be described below. You may ignore the Ch#
identifications since they are filter position assignments for the
proposed ground-based system.)

The best match is FC8 (at 425 nm) and B-band, but FC8 is narrower,
and this means that a target with a spectral slope (flux vs.
wavelength) will have a different effective wavelength for the two
bands. The same applies to FC2 and V-band. The other possible
matches are even worse because effective wavelengths will have a
greater dependence upon spectral slope. If a target's spectral slope
is known then in theory corrections can be made, but each target
with a different spectral slope will require a separate correction.

The same comments, above, apply to any attempt to use star SDSS
magnitudes for calibrating the FC.

The two graphs below show relative probability of photons incident
at the top of the atmosphere producing photo-electrons (i.e.,
includes losses due to atmospheric extinction, telescope optical
transmission and CCD QE). I'll refer to this as a filter band's
"responsivity."

FC band responsivity (photo-electron production) and Vega
spectrum.

FC band responsivity (photo-electron production) and solar
spectrum. Ground-Based Support for Using Asteroid
Flux Measurements as a Check on Dawn FC Albedo Measurements

Both Vesta and Ceres have
disk-averaged measurements (total Data Number, DN) for each FC
filter. This information is for specific phase angle viewing
geometries. If actual albedos for Vesta and Ceres were known as
a function of phase angle for each FC filter then the Dawn FC
measurements could be adjusted to agree with "ground-truth." By
"ground-truth" is meant determination of albedo using
ground-based observations with an identical set of FC filters,
as a function of phase.

The goal of the proposed
ground-based observations is disk-average fluxes [watts/m2/um]
for a suite of observations that sample a range of viewing
geometries. Fluxes can be compared with solar incident flux for
each observing situation to determine albedo (for that FC band
and at that phase angle). These data can be modeled, and
predictions can be made for any Dawn disk-average DN, for the
specific FC band and viewing geometry that applies. Any required
adjustment to a FC channel will appear as a persistent
coefficient that should be multiplied to the default Dawn FC
flux in order to achieve agreement with the ground-based model
flux.

The ground-based observations
will require the creation of new magnitude systems for each FC
band. Vega will be used as a primary standard for establishing
zero magnitude (above the atmosphere). Secondary standard stars
will be calibrated using Vega: two A0V stars (same as Vega) and
several sun-like stars. These secondary stars will be located
close to the position of Vesta and Ceres for their 2014
oppositions (they're close together in the sky, so their
oppositions are in mid-April). This will permit easy calibration
of Vesta and Ceres using the nearby (same air mass) secondary
standards. A description of how this will be accomplished is in
a white paper, "Deriving a Magnitude System," available from
Bruce Gary (this web site's webmaster). All measurements will be
made with a Celestron 11-inch (CPC1100), with a 10-position CFW
and SBIG ST-10XME CCD, all inside a dome in my backyard that is
controlled from inside my house.

Progress reports can be found at: index.html
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